feat(auth): throttle login, register, password reset and Subsonic auth failures (M462 #4976)

Every password-shaped check was mounted bare, so guessing was limited only
by bcrypt cost. A shared in-memory AttemptLimiter now sits in front of them:

- login: 10 failures per account and 50 per address per 15 min, checked
  before the user lookup and bcrypt; 429 with Retry-After. A success clears
  the account's count but not the address's.
- unknown usernames run a dummy bcrypt compare, so timing no longer says
  which accounts exist.
- register: 10 per address per hour; forgot-password: 5 per address and 3
  per email per hour (applied whether or not the email matches); reset: 20
  failed tokens per address per 15 min.
- Subsonic /rest: same limits as login, counting only wrong credentials,
  since clients authenticate on every request.

Web login, register, reset and forgot-password screens say how long to
wait; web and Android carry copy for the rate_limited code.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/api.go

@@ -7,6 +7,7 @@ package api
 import (
 	"log/slog"
 	"math/rand"
+	"time"
 
 	"github.com/go-chi/chi/v5"
 	"github.com/jackc/pgx/v5/pgxpool"
@@ -58,6 +59,11 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 		reacqSettings:       reacqSettings,
 		fingerprintSettings: fpSettings,
 		librarySize:         recommendation.NewLibrarySize(nil),
+		loginGuard:          auth.NewLoginGuard(),
+		registerLimit:       auth.NewAttemptLimiter(registerPerAddressMax, time.Hour),
+		forgotAddressLimit:  auth.NewAttemptLimiter(forgotPerAddressMax, time.Hour),
+		forgotEmailLimit:    auth.NewAttemptLimiter(forgotPerEmailMax, time.Hour),
+		resetLimit:          auth.NewAttemptLimiter(resetFailuresPerAddressMax, 15*time.Minute),
 	}
 
 	r.Route("/api", func(api chi.Router) {
@@ -313,6 +319,14 @@ type handlers struct {
 	// instance the scanner and the fingerprint workers read, so a save from the
 	// admin card reaches them without a restart. Nil serves the defaults.
 	fingerprintSettings *library.FingerprintSettingsService
+	// loginGuard throttles failed logins per account and per address, and
+	// the limiters below cap the other unauthenticated auth routes. All are
+	// nil-safe, so tests that build handlers directly run unthrottled.
+	loginGuard         *auth.LoginGuard
+	registerLimit      *auth.AttemptLimiter
+	forgotAddressLimit *auth.AttemptLimiter
+	forgotEmailLimit   *auth.AttemptLimiter
+	resetLimit         *auth.AttemptLimiter
 	// netSettings caches the trusted reverse-proxy depth read by the auth
 	// middleware on every request and edited from the admin network card.
 	netSettings *netsettings.Service

internal/api/auth.go

@@ -19,6 +19,20 @@ import (
 // so an abandoned laptop doesn't stay logged in forever.
 const sessionCookieMaxAge = 30 * 24 * time.Hour
 
+// Limits on the unauthenticated auth routes other than login (which uses
+// auth.LoginGuard). Per address, per window as wired in Mount.
+const (
+	// registerPerAddressMax: 10 registrations an hour. Every attempt counts,
+	// typos included, which is still far above a household's need.
+	registerPerAddressMax = 10
+	// forgotPerAddressMax / forgotPerEmailMax: reset emails an hour. The
+	// per-email cap is what keeps one inbox from being mailbombed.
+	forgotPerAddressMax = 5
+	forgotPerEmailMax   = 3
+	// resetFailuresPerAddressMax: wrong or expired reset tokens per 15 min.
+	resetFailuresPerAddressMax = 20
+)
+
 func (h *handlers) handleLogout(w http.ResponseWriter, r *http.Request) {
 	// The session token can be on the cookie OR bearer header — RequireUser
 	// accepted either. Re-resolve it here so we can delete the row.
@@ -70,10 +84,22 @@ func (h *handlers) handleLogin(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Checked before any lookup or bcrypt, so a throttled guess costs the
+	// server nothing and learns nothing.
+	addr := auth.ClientIP(r, h.netSettings.Hops())
+	if blocked, wait := h.loginGuard.Blocked(req.Username, addr); blocked {
+		writeRateLimited(w, wait)
+		return
+	}
+
 	q := dbq.New(h.pool)
 	user, err := q.GetUserByUsername(r.Context(), req.Username)
 	if err != nil {
 		if errors.Is(err, pgx.ErrNoRows) {
+			// Same bcrypt time as a wrong password, so timing doesn't say
+			// which usernames exist.
+			auth.DummyVerify(req.Password)
+			h.loginGuard.Fail(req.Username, addr)
 			writeErr(w, apierror.Unauthorized("invalid_credentials", "invalid username or password"))
 			return
 		}
@@ -82,9 +108,11 @@ func (h *handlers) handleLogin(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if !auth.VerifyPassword(user.PasswordHash, req.Password) {
+		h.loginGuard.Fail(req.Username, addr)
 		writeErr(w, apierror.Unauthorized("invalid_credentials", "invalid username or password"))
 		return
 	}
+	h.loginGuard.Succeed(req.Username)
 
 	token, err := auth.MintSessionToken()
 	if err != nil {
@@ -100,7 +128,7 @@ func (h *handlers) handleLogin(w http.ResponseWriter, r *http.Request) {
 		// the active-sessions surface: a session that was born somewhere the
 		// user recognises but is being used from somewhere they don't is the
 		// case this whole surface exists to surface.
-		Ip: auth.ClientIP(r, h.netSettings.Hops()),
+		Ip: addr,
 	}); err != nil {
 		h.logger.Error("api: insert session failed", "err", err)
 		writeErr(w, apierror.InternalMsg("insert failed", err))

internal/api/auth_forgot.go

@@ -14,6 +14,7 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/audit"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/auth"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/mailer"
 )
@@ -47,6 +48,19 @@ func (h *handlers) handleForgotPassword(w http.ResponseWriter, r *http.Request)
 	}
 	email := strings.ToLower(strings.TrimSpace(req.Email))
 
+	// Throttled per address (a spray) and per email (a mailbomb of one
+	// inbox). Applied whether or not the email matches, so a 429 says
+	// nothing about which addresses are registered.
+	addr := auth.ClientIP(r, h.netSettings.Hops())
+	blockedAddr, waitAddr := h.forgotAddressLimit.Blocked(addr)
+	blockedEmail, waitEmail := h.forgotEmailLimit.Blocked(email)
+	if blockedAddr || blockedEmail {
+		writeRateLimited(w, max(waitAddr, waitEmail))
+		return
+	}
+	h.forgotAddressLimit.Record(addr)
+	h.forgotEmailLimit.Record(email)
+
 	q := dbq.New(h.pool)
 	var matched bool
 	var auditTarget pgtype.UUID

internal/api/auth_register.go

@@ -55,6 +55,15 @@ type registerReq struct {
 //   - 409 username_taken (PG unique violation on users.username)
 //   - 500 server_error otherwise
 func (h *handlers) handleRegister(w http.ResponseWriter, r *http.Request) {
+	// Every attempt counts, not only failures: a successful registration is
+	// the thing being sprayed when the mode is open.
+	addr := auth.ClientIP(r, h.netSettings.Hops())
+	if blocked, wait := h.registerLimit.Blocked(addr); blocked {
+		writeRateLimited(w, wait)
+		return
+	}
+	h.registerLimit.Record(addr)
+
 	var req registerReq
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		writeErr(w, apierror.BadRequest("invalid_body", "invalid JSON body"))
@@ -175,7 +184,7 @@ func (h *handlers) handleRegister(w http.ResponseWriter, r *http.Request) {
 		UserID:    user.ID,
 		TokenHash: auth.HashSessionToken(sessionToken),
 		UserAgent: r.UserAgent(),
-		Ip:        auth.ClientIP(r, h.netSettings.Hops()),
+		Ip:        addr,
 	}); err != nil {
 		h.logger.Error("register: insert session failed", "err", err)
 		writeErr(w, apierror.Internal(err))

internal/api/auth_reset.go

@@ -10,6 +10,7 @@ import (
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/audit"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/auth"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 )
 
@@ -42,6 +43,15 @@ func (h *handlers) handleResetPassword(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Tokens carry 256 bits, so guessing one is hopeless; the limit is on
+	// failed tries per address so that nobody gets to find that out at our
+	// expense.
+	addr := auth.ClientIP(r, h.netSettings.Hops())
+	if blocked, wait := h.resetLimit.Blocked(addr); blocked {
+		writeRateLimited(w, wait)
+		return
+	}
+
 	q := dbq.New(h.pool)
 
 	// Look up the reset record so we know which user to update before
@@ -50,6 +60,7 @@ func (h *handlers) handleResetPassword(w http.ResponseWriter, r *http.Request) {
 	reset, err := q.GetPasswordReset(r.Context(), req.Token)
 	if err != nil {
 		if errors.Is(err, pgx.ErrNoRows) {
+			h.resetLimit.Record(addr)
 			writeErr(w, apierror.BadRequest("invalid_token", ""))
 			return
 		}
@@ -69,6 +80,7 @@ func (h *handlers) handleResetPassword(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if rows == 0 {
+		h.resetLimit.Record(addr)
 		writeErr(w, apierror.BadRequest("invalid_token", ""))
 		return
 	}

internal/api/auth_test.go

@@ -179,6 +179,36 @@ func TestHandleLogin_UnknownUserReturns401(t *testing.T) {
 	}
 }
 
+// A guesser who reaches the account limit is refused with 429 before any
+// password check, so even the right password is turned away until the window
+// passes, and the response says when to come back.
+func TestHandleLogin_ThrottledAfterRepeatedFailures(t *testing.T) {
+	h, pool := testHandlers(t)
+	h.loginGuard = auth.NewLoginGuard()
+	seedUser(t, pool, "alice", "hunter2", false)
+
+	login := func(password string) *httptest.ResponseRecorder {
+		body := strings.NewReader(`{"username":"test-alice","password":"` + password + `"}`)
+		req := httptest.NewRequest(http.MethodPost, "/api/auth/login", body)
+		req.Header.Set("Content-Type", "application/json")
+		w := httptest.NewRecorder()
+		h.handleLogin(w, req)
+		return w
+	}
+	for i := 0; i < 10; i++ {
+		if w := login("wrong"); w.Code != http.StatusUnauthorized {
+			t.Fatalf("attempt %d: status = %d, want 401", i+1, w.Code)
+		}
+	}
+	w := login("hunter2")
+	if w.Code != http.StatusTooManyRequests {
+		t.Fatalf("status = %d, want 429 once the account limit is reached", w.Code)
+	}
+	if w.Header().Get("Retry-After") == "" {
+		t.Error("429 without Retry-After")
+	}
+}
+
 func TestHandleLogin_MalformedBodyReturns400(t *testing.T) {
 	h, _ := testHandlers(t)
 	req := httptest.NewRequest(http.MethodPost, "/api/auth/login",

internal/api/errors.go

@@ -3,7 +3,10 @@ package api
 import (
 	"encoding/json"
 	"log/slog"
+	"math"
 	"net/http"
+	"strconv"
+	"time"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
 )
@@ -36,6 +39,17 @@ func writeErr(w http.ResponseWriter, err error) {
 	}})
 }
 
+// writeRateLimited answers 429 with Retry-After in whole seconds, rounded
+// up so a client that honours it never arrives a moment early.
+func writeRateLimited(w http.ResponseWriter, wait time.Duration) {
+	secs := int(math.Ceil(wait.Seconds()))
+	if secs < 1 {
+		secs = 1
+	}
+	w.Header().Set("Retry-After", strconv.Itoa(secs))
+	writeErr(w, apierror.TooManyRequests("rate_limited", "too many attempts; try again later"))
+}
+
 // writeErrWithLog logs the error at Error level and writes the response.
 // Use for 500-class errors where the operator needs the cause in logs.
 func writeErrWithLog(w http.ResponseWriter, logger *slog.Logger, msg string, err error) {